When being abroad it is a common problem for the travellers to be able to estimate and/or calculate the prices in their own home currency they are more familiar with. To facilitate this various exchange rate scales or tables and similar simple mechanical means and electronic calculators, even programmable ones, are known.
These do, however, have some disadvantages. The tables and similar tend to be out of date quite soon due to fluctuations in exchange rates. The same is true for the calculators having reprogrammed exchange rates stored in the memory means thereof. A further problem relies on the fact that the traveller needs to carry such table, scale or calculator with him or her, which means one further thing to remember to take with and to take care of.
Due to the fact that mobile telephones have become and are still becoming more and more popular, even the travellers, and especially those travelling for business purposes do have a mobile station with them when they are abroad, as it is even possible to use the same mobile telephone or station when being abroad as is used at home. Analog and digital mobile telephones as well as mobile telephone networks and apparatus and equipment belonging to those are well known by the skilled person, and thus not explained in more detail. In this description the term `network` in general means all such means required to run a network, and thus it is intented to include, among other things, operators, exchanges, switches, base stations and similar. The term `home network` means the network in which the user is a subscriber. The term `local network` means the network in which the user is at the present, and may be either the home network or a foreign network.
ETSI (European Telecommunications Standards Institute) specification GSM 02.86 (ETS 300 519: European digital telecommunications system (Phase 2); Advise of Charge (AoC) supplementary services--Stage 1), December 1995, discloses main principles and features of an `Advice of Charge` (AoC) service and is incorporated herein by reference.
Said AoC service provides the subscriber with an information indicating an estimated cost of a call or a service used, ie. a cost for every applicable usage of telecommunications services. The AoC service is subscribed from a service provider, such as from the home network operator, who activates said AoC service.
The users have found this service useful when they are estimating the actual costs caused by the usage of the mobile telephone. The costs are always expressed in home units, the value of which is defined and published by the HPLMN (Home Public Land Mobile Network) regardless of the LPLMN (Local Public Land Mobile Network) in which the costs were incurred or regardless of the call direction or regardless of the type of call including supplementary service and data calls. In other words, this service is not capable of making any calculations between various currencies.
Another ETSI specification GSM 02.24 (ETS 300 510: European digital cellular tele-communications system (Phase 2); Description of Charge Advise Information (CAI)), August 1995, and ETSI specification GSM 04.86 (ETS 300 570: European digital cellular telecommunications system (Phase 2); Advise of Charge (AoC) supplementary services--Stage 3), November 1994, describe a `Charge Advice Information` (CAI) service. These specifications are also incorporated herein as a reference.
The CAI service provides several additional features to the AoC, such as more precise information about the calls and the costs. The mobile telephone or mobile station is supplied with the necessary CAI information at the charging point on a per call basis within a signalling message over the radio interference. The mobile telephone uses the CAI information to compute the AoC value for the relevant call. Thus the signalling CAI not only provides the charging information, but indicates also the charging point and hence initiates the timing of the chargeable duration. According to this specification the information sent to the mobile telephone consists of seven elements as is disclosed in the following table:
TABLE 1 ______________________________________ Element Dimension Description ______________________________________ e1 u/i Units / interval e2 t/i Seconds / time interval e3 -- Scaling factor e4 u Unit increment e5 u/d Units / data interval e6 seg/d Segments / data interval e7 t/i initial seconds / time interval ______________________________________
From the above the element e3 defines a scaling factor for conversions from LPLMN units to HPLMN units, said element e3 being a dimensionless multiplier. The e3 is set according to variables of the LPLMN and the relevant HPLMN. If the LPLMN is the HPLMN, ie. the mobile telephone is in the home network, e3 will be simplified to unity. The e3 element of the CAI allows the mobile telephone to calculate the costs of a call in a currency chosen by the subscriber. The specification defines also a PUCT (Price per Unit and Currency Table), which is the value of the home unit in a currency chosen by the subscriber and being stored in a SIM (Subscriber Identity Module) of the mobile telephone. The subscriber may set the currency value corresponding the value of the home unit differently in the PUCT to the value of the unit published by the HPLMN in order to cover extra cost. An indication of the currency is part of the PUCT.
However, the above services do not give any help in view of prices or costs other than caused by making a telephone call or using other services provided by means of the mobile station.